Method for the centrifugal production of metal tubes

ABSTRACT

A method for the centrifugal production of a metal tube having a composition which varies within the thickness of the wall of the tube, comprising the steps of pouring a mass of molten metal in a rotary mould then, before the complete solidification of said mass of metal, depositing on the mass of metal in the mould a molten slag containing addition elements, allowing the metal and the slag to solidify while continuing to rotate the mould until solidification of both of them, and removing the solidified slag.

United States Patent 1191 Royer 1 June 4, 1974 METHOD FOR THE CENTRIFUGAL 2,745,740 5/1956 Jackson 164/114 ux PRODUC'HON 0F METAL TUBES 3,293,708 12/1966 Fruitman 164/1 14 3,563,300 2/1971 Honda et a]. 164/! 14 x [75] lnventor: Alain Louis Albert Royer, Fumel,

France [73] Assignee: Pont-A-Mousson S.A., Nancy, I

France [22] Filed: Sept; 13, 1972 21 Appl. No.: 288,824

[30] Foreign Application Priority Data Sept. 24, 197i Francc....- 7l.3438() [52] US. Cl 164/114, 164/58, 164/55 [51] Int. Cl 822d 13/00 [58] Field of Search, l64/5 5,5 8, 57, 114

[56] References Cited UNITED STATES PATENTS 1.614.863 l/l927 Beatty l64/ll4 1.678.931

7/1928 Blumberg et al. 164/! 14 X Primary Examiner.l. Spencer Overholser Assistant ExaminerV. K. Rising 4 Attorney, Agent, or FirmSughrue, Rothwell, Mion,

Zinn & Macpeak [57] ABSTRACT A method for the centrifugal production of a metal tube having a composition which varies within the 7 Claims, 2 Drawing Figures METHOD FOR THE CENTRIFUGAL PRODUCTION OF METAL TUBES The present invention relates to a method for the centrifugal production of metal tubes the wall of which has a varying composition.

It is advantageous or necessary in some cases to produce tubes having coaxial cylindrical layers which afford different chemical, physical and mechanical properties. For example, it may be desired that the inner surface layer have high hardness so as to resist abrasion whereas a lower hardness is required for an outer layer but with no brittleness while achieving between the layers, notwithstanding their differences, maximum cohesion to preclude formation of a film of oxide therebetween.

It is known to produce metal tubes by the successive centrifugal casting of two layers from two metals or alloys having different compositions. However, this.

method requires the use of two processing furnaces and, furthermore, the ratio between the thicknesses of the two layers may vary undesirably along a generatrix.

It is also known to produce tubes having a variable composition within the thickness of their wall by quenching the inner surface of a previously-heated metal tube by means of jets. This presupposes that the initial metal or alloy is suitably capable of being quenched in this way and, moreover, the internal stresses produced in the resulting tube by such an operation may render the tube unsuitable for some conditions of pressure in use.

An object of the present invention is to avoid all these drawbacks by means of a centrifugal production method which puts to use the laws of diffusion of chemical elements in liquid bodies in the course of solidification.

The invention provides a method for the centrifugal production of a metal tube having a varying composition within the thickness of its wall, comprising pouring a mass of molten metal in a rotary mould then, before the complete solidification of said mass of metal, depositing on the mass of metal in the mould a molten slag containing addition elements, allowing the metal and the slag to solidify while continuing to rotate the mould until solidification of both of them, and removing the solidified slag.

The diffusion of the addition elements contained in the slag in the inner surface layer of the mass of molten metal modifies the composition and structure of said layer, so that there is obtained a metal part having a composition which varies within the thickness of its wall, the inner layer of the'tube being enriched with the addition elements. The thickness of the inner layer and its content of addition elements depend on the time which elapses'between the pouring of the metal and the 1 pouring of the slag, on the composition of the slag, on the respective temperatures of the metal and slag at the moment of the pouring and on the speed of rotation of the mould. These parameters must therefore be chosen experimentally in accordance with requirements.

In a particular manner of carrying out the method, the metal is steel and the slag is a slag which isrich with graphite in suspension. In this case, the inner layer is enriched with carbon which has a case-hardening effeet.

A slag having a low density is preferably chosen so as to avoid any risk in variation in the thickness of the two layers throughout the length of the tube.

EXAMPLE I.

To produce a metal tube (diameter mm X 200 mm and length 6 m) such as that shown in FIG. 1 a 18 CDV4 steel is taken having the following composition (percent by weight) according to a test or sample castmg:

c 0.18 P 0.012 Cr 0.99 Si 0.26 Mn 1.09 MO 0.33 5 0.006 Ni 0.06 Va This steel is poured into a centrifugal casting mould which rotates at a speed giving an acceleration of .80 g (being the acceleration of gravity) then, 1 minute 10 seconds after the end of the pouring of the steel, that is, well before its solidification, a slag is poured which has an appropriate composition specially charged with graphite and the mould is continued to be rotated until solidification of the poured materials. Thereafter, the tube is withdrawn from the mould and the internal crust of solidified slag is removed.

The structure of the tube obtained, as seen in section, is shown in FIG. 1. The latter shows a section made after polishing and etching with ammonium persulfate. Analysis shows that the innersurface layer 1 having a thickness of 9 mm has'a carbon content exceeding 0.20 percent whereas the layer 2 which surrounds it has a content limited to 0.20 percent. The separation is perfectly clean and definite between the two layers.

FIG. 2 shows the variation in the carbon content of the tube as a function of the depth of the sample from the innermost layer of the tube, that is to say, the distance from the axis of the tube.

The values are as follows:

diameter in mm carbon content I55 [.60 I63 I 0.88 17] 0.20 I79 (H7 187 0.15 0.17 203 0.18 211 0.14 219 (M3 It can be seen that this content increases in the direction of the inner wall which was in contact with the graphitizing slag and was therefore subjected to the diffusion of the carbon. It can also be seen that this content in the outer layer slightly decreases from 0.20 percent, the mean content being naturally that of the start, and the gradiant conforms to the usual result of centrifugal production of low alloy steels.

There has also been plotted in the figure a mean curve and the found contents deviate from the values of this curve to an extent which corresponds to normal errors of analysis. This curve shows still more clearly the large increase in the carbon content in the surface layer.

Further, the micrographic examinations shown, after Nital etching, an inner layer having pearlite-cementite constituents, an intermediate transition layer of several metal tube having a composition which varies within the thickness of its wall and practically having two layers.

EXAMPLES 2 to 4 In the same way, the method is employed to obtain inner layers enriched with nitrogen, sulphur and boron. The nitrogen is of interest for its hardness qualities (1,200 l-Iv in respect of certain chromium or niobium nitrides), the inner nitrided layer obtained having in addition a low coefficient of friction; the sulphur and the boron can afford machinability or participate in the solving of certain problems of friction. In the first case, a slag charged with calcic cyanamideis employed.

EXAMPLES 5 to 7 The method is employed in the same manner to obtain diffusion of silicon, nickel and chromium.

The method is therefore applicable in a general way to any action of a slag adding a metallic or metalloid element or elements to a metal in the course of solidification, on the sole condition that the metal receiving the element has the desired ability to fix the element and that the slag has a complementary composition adapted to perform the function of a vehicle for this element.

Having now described my invention what I claim as new and desire to secure by Letters Patent is:

1. A method for the centrifugal production of a metal tube having a composition which varies within the thickness of the wall of the tube, comprising the steps of pouring a single casting in a rotary mould of a single mass of molten metal having a composition corresponding to that required for the outer layer of the tube then, before the complete solidification of said mass of metal, depositing on the mass of metal in the mould a molten slag containing additional elements for superficial diffusion in said metal, said metal being capable of fixing said addition elements, diffusing said additional elements in an inner layer of said metal by allowing said metal and said slag to solidify while continuing to rotate the mould until solidification of both of them, and removing the solidified slag devoid of the addition elements which have diffused in said metal and produced said variation in the composition.

2. A' method as claimed in claim 1, wherein the metal is steel;

3. A method as claimed in claim 1, wherein the slag is charged with metalloid.

4, A method as claimed in claim 3, wherein the slag is a slag charged with carbon.

5. A method as claimed in claim 4, wherein the slag is a slag rich with graphite in suspension.

6. A method as claimed in claim 3, wherein the slag is a slag charged with nitrogen.

7. A method as claimed in claim 1 wherein the slag is a slag charged with a metallic element. 

1. A method for the centrifugal production of a metal tube having a composition which varies within the thickness of the wall of the tube, comprising the steps of pouring a single casting in a rotary mould of a single mass of molten metal having a composition corresponding to that required for the outer layer of the tube then, before the complete solidification of said mass of metal, depositing on the mass of metal in the mould a molten slag containing additional elements for superficial diffusion in said metal, said metal being capable of fixing said addition elements, diffusing said additional elements in an inner layer of said metal by allowing said metal and said slag to solidify while continuing to rotate the mould until solidification of both of them, and removing the solidified slag devoid of the addition elements which have diffused in said metal and produced said variation in the composition.
 2. A method as claimed in claim 1, wherein the metal is steel.
 3. A method as claimed in claim 1, wherein the slag is charged with metalloid.
 4. A method as claimed in claim 3, wherein the slag is a slag charged with carbon.
 5. A method as claimed in claim 4, wherein the slag is a slag rich with graphite in suspension.
 6. A method as claimed in claim 3, wherein the slag is a slag charged with nitrogen.
 7. A method as claimed in claim 1, wherein the slag is a slag charged with a metallic element. 